In the assay of a sample for the presence or absence of a certain condition or a particular analyte, reagents/components of the assay system are often added at different stages in the assay, i.e., they are not combined until the appropriate stage in the assay. The particular order in which reagents are combined is determined by the requirements of the particular assay. For example, in some systems it is necessary to incubate to bring the sample and/or reagents to a desired temperature before initiating the assay; in two step assays wherein one reaction must precede the other, if reagents for the second reaction interfere with the first reaction then these reagents must not be added until after the first reaction is complete; and so on.
Assays wherein components/reagents are added or combined at various times during the course of the assay are subject to user error and are often cumbersome and inefficient to run. One assay developed to measure the condition of the blood of a patient is a case in point as will be discussed hereinafter.
Hemostasis or stoppage of bleeding involves the interplay of two biochemical pathways which are controlled by various protein factors and formed elements, e.g., platelets. The processes by which blood coagulates as it is presently understood involve a multi-step cascade of activations of the protein factors that culminate in fibrin formation. Various tests have been developed to test the individual steps of this cascade in order to determine whether the blood of a patient can properly clot or whether there is clotting disorder in which there is a deficiency of one or more of the factors necessary for proper clotting. It is well known that the condition of the platelets or the platelet function of blood is one indication of the ability of blood to properly clot.
The primary existing test in use for testing platelet function or Primary Hemostasis on whole human blood is known as the bleeding time test. The bleeding time test which has existed for several decades involves an incision on the forearm of the patient. Accordingly, a test which does not involve an incision and which is also more accurate was developed.
U.S. Pat. Nos. 4,604,894; 4,780,418; and 5,051,239 disclose an assay system which can be used to perform an in vitro test on blood that can be accurately and reproducibly correlated to the in vivo bleeding time test described above, thereby eliminating involvement of the patient. The Thrombostat.TM. 4000 (Baxter Diagnostics), in current use, is one such system. Platelet function is evaluated in these systems by aspirating anticoagulated whole blood samples at a constant negative pressure through a small aperture positioned at the center of a separating wall which may be non-porous or porous. In systems wherein the separating wall is porous, it is wetted prior to the start of the assay with an activator that activates coagulation of blood platelets. A platelet plug forms at the aperture and the time required for the cessation of blood flow to occur is determined. This time is then correlated to platelet function, i.e., in vivo bleeding time.
The Thrombostat.TM. 4000 system is not in widespread use, due largely to the present configuration which is costly and does not lend itself to automation for a number of reasons, including limitations of the device which holds the sample to be tested. The device currently used with the Thrombostat.TM. 4000 consists of three separate parts: a reagent/test chamber, a capillary, and a sample cup. A porous separating wall containing collagen is disposed in the reagent/test chamber. The reagent/test chamber then must be stored in a separate hermetic package apart from the capillary and sample cup to maintain stability of the collagen for the specified shelf life. The capillary and reagent/test chamber must be manually assembled by the operator at the start of each test being performed. Furthermore, the sample to be tested must be pipetted into the sample cup and incubated before the sample cup can be assembled to the capillary and reagent/test chamber. In addition, the incubation step is manually timed by the operator. The separate incubation step requires additional handling after the incubation period, when the operator manually places the assembled capillary and reagent/test chamber into the sample cup and initiates the testing sequence. At the end of the test, the capillary is removed and cleaned for reuse because of its high cost.
It can be seen that the Thrombostat.TM. 4000 system could be improved by use of a device which eliminates the need for user interaction during a test cycle, which does not require complicated sample handling mechanisms, which eliminates the need for a separate external hermetic package for the reagent/test chambers during shipping and storage and which is disposable. A device which accomplishes these objectives would be generally useful in assay systems wherein certain components/reagents are kept separated or not combined until the appropriate time. Accordingly, such devices are being sought.